The present invention relates to an article comprising a substrate containing silicon and a barrier layer which functions as a protective environmental and/or thermal barrier coating which inhibits the formation of gaseous species of Si, particularly Si(OH)x when the article is exposed to a high temperature, aqueous (water and/or steam) environment and, more particularly, a barrier layer having an additive component which is capable of forming a reaction product with free silica thereby prohibiting a reaction between free silica and the barrier coating.
Ceramic materials containing silicon and metal alloys containing silicon have been proposed for structures used in high temperature applications as, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particularly useful application for these materials is for use in gas turbine engines which operate at high temperatures in aqueous environments. It has been found that these silicon containing substrates can recede and lose mass as the result of formation of volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200xc2x0 C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs. It is believed that one possible reaction mechanism involves oxidation of the silicon carbide to form silica and carbon monoxide and/or carbon dioxide on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x. Other mechanisms are possible such as direct oxidation of SiC to Si(OH)x species by steam. A barrier coating for silicon containing substrates which would form a reaction product with the silica created during the oxidation of the silicon carbide and thus inhibit the formation of volatile species of silicon, namely, Si(OH)x and SiO, is known. In accordance with the invention, an additive is made to the barrier layer which additive reacts with any free silica. This prohibits reaction between silica and the barrier layer which results in a low melting point problem. Free silica occurs from two sources: (1) deposits on the surface of the barrier layer from the environment, i.e. sand, and (2) because the barrier layer works well, oxidation of the silicon containing substrate leads to silica which is stable because the barrier layer prevents water from getting to that surface.
Accordingly, it is the principle object of the present invention to provide an article comprising a silicon containing substrate with a barrier layer which includes an additive component capable of forming a reaction product with free silica and thereby inhibit the formation of reactions between the barrier layer components and silica.
Further objects and advantages of the present invention will appear hereinbelow.
The present invention relates to an article comprising a silicon containing substrate having a barrier layer on the substrate, wherein the barrier layer functions to both inhibit the formation of undesirable gaseous species of silicon when the article is exposed to a high temperature, aqueous environment and to provide thermal protection such as exists in a gas turbine combustion environment. By high temperatures is meant the temperature at which the Si in the substrate forms Si(OH)x and/or SiO in an aqueous environment. By aqueous environment is meant a water and/or steam environment. The silicon containing substrate is preferably a ceramic or metal alloy containing silicon. The barrier layer in accordance with the present invention comprises an aluminosilicate selected from the group consisting of barium aluminosilicate, strontium aluminosilicate and barium strontium aluminosilicate and an additive component capable of forming a reaction product with free silica. It is preferred in accordance with the present invention that the aluminosilicate is alkaline earth based such as barium strontium aluminosilicate comprising 0.75 BaO.0.25 SrO.Al2O3.2SiO2. The additive component in accordance with the present invention is selected from the group consisting of Al2O3, BaO.6Al2O3, BaO.Al2O3, SrO.6Al2O3, SrO.Al2O3 and mixtures thereof. The alkaline earth based aluminosilicate plus additive can also be referred to as non-stoichiometric BAS, BSAS, or SAS in that it is sub-stoichiometric with respect to silica. The barrier layer in accordance with the present invention has an increased capacity for silica over barrier layers heretofore known. By the addition of the additive component, roughly 32 more silica may be tolerated when compared to a barium strontium aluminosilicate barrier layer without the additive component.